As part of the National HIV Behavioral Surveillance System among men who have sex with men (MSM) in Denver, Colorado, we assessed knowledge of pre-exposure prophylaxis (PrEP); willingness to use PrEP; and potential changes in risk behaviors among HIV-negative participants reporting sexual activity with a male partner in the preceding 12 months. We examined knowledge of PrEP before (2008) and after (2011) results of the iPrEx trial were available. Of the 425 participants in the 2008 sample, 91 (21 %) were aware of PrEP compared to 131 (28 %) of the 461 participants in the 2011 sample (adjusted prevalence ratio: 1.43, 95 % confidence interval: 1.18, 1.72). Despite the increase in 2011, few MSM in Denver were aware of PrEP. Educating high-risk MSM about the potential utility of PrEP as an adjunct to other effective prevention methods is needed when considering the addition of PrEP to the HIV prevention arsenal.